| The formation of orogens is a prominent feature in convergent plate margins, and study on evolutions of orogens is one of key topics in continental dynamics. More and more studies have revealed that prolonged multistage accretions of island arcs and/or microcontinents occurred before the final continental collision between two large plates. This process is accompanied with growth and reworking of continental crust. The Qinling-Tongbai-Dabie-Sulu orogenic belt is a typical composite orogen, and its formation recorded the multistage amalgamation between the South and North China blocks. Compared to the eastern Dabie-Sulu orogen, the western Qinling-Tongbai orogen registers entire records of the early Paleozoic evolution and significant crustal growth. However, comprehensive and intensive studies on the Qinling-Tongbai orogen are absent. In this contribution, we carried out an integrated study of U-Pb isotopes, mineral inclusions, trace elements, and Hf-O isotopes on zircons as well as whole-rock major and trace elements and Sr-Nd-Pb isotopes on some key samples in the Qinling-Tongbai orogen. The main points of the results are summaried as follows:(1) U-Pb age determinations on magmatic zircon cores of two eclogite samples yield similar U-Pb ages of798±23and799±94Ma, indicating that the protoliths of the North Qinling (NQ) UHP eclogites formed in the mid-Neoproterozoic. The NQ eclogites have tholeiitic compositions and are characterized by high TiO2(1.40-2.40%), and low P2O5(0.09-0.22%), Na2O (1.10-2.49%) and K2O (0.17-1.04%) contents. They are moderate fractionation in REEs (LaN/YbN=2.31-4.20) and show negligible Nb, Ta, Ti, and Eu anomalies (Eu/Eu*=0.82-1.10). The eclogites have positive whole-rock εNd(t)(2.81-5.53) and zircon εHf(t)(10.5-11.9) values, indicating their derivation from a relatively depleted mantle source. On tectonic discriminant diagrams based on immobile element ratios, the NQ eclogites fall in the fields of continental basalts. Moreover, both the geochronological and geochemical features of the eclogite protoliths are quite different from those of the Erlangping basaltic rocks, but comparable to those of the coeval continental basalts in the South Qinling (SQ) orogen and the northern margin of the South China Block (SCB). It is inferred that the NQ eclogite precursors might also form in a continental rift setting at ca.800Ma in response to the global-scale breakup of the Rodinia supercontinent, which may result in the splitting of the NQ terrane from the SCB.(2) The metamorphic zircon rims in an eclogite sample show no zoning or weak zoning, very low Th/U and176Lu/177Hf ratios, insignificant Eu anomalies and flat HREE patterns. They contain inclusions of garnet, omphacite and phengite, suggesting that the metamorphic zircon formed under eclogite-facies metamorphic conditions, and their weighted mean206Pb/238U age of486±4Ma was interpreted to date the timing of the eclogite-facies metamorphism. Zircon in a quartz vein is characterized by perfect euhedral habit, some oscillatory zoning, low Th/U ratios and variable HREE contents. It yields a weighted mean U-Pb age of481±3Ma, which registers the age of fluid activity during exhumation. Zircon in the schist is mostly detrital and U-Pb age peaks at c.1950-1850,1800-1600,1560-1460and1400-1260Ma with an oldest grain of2517Ma, also suggesting that the NQ terrane may have an affinity to the SCB. Accordingly, the amalgamation between the South and North China blocks is a multistage process, which includes: the formation of the Erlangping intra-oceanic arc zone onto the North China Block (NCB) before ca.490Ma, the ca.490Ma crustal subduction and UHP metamorphism of the NQ terrane, and ca.480Ma fluid activity during exhumation.(3) The zircon crystals in an amphibolite sample show typical metamorphic growth zoning, low Th/U ratios, flat HREE patterns, and insignificant Eu anomalies. They give a weighted mean U-Pb age of490±6Ma. Most importantly, an in situ diamond inclusion was identified from one of the zircon crystals. Therefore, the obtained490±6Ma age was taken as registering the peak UHP metamorphism. The discovery of in situ diamond inclusion provides important evidence for the UHP metamorphism of the Qinling orogen and corroborates that the NQ microcontinent subducted to mantle depths of>120km when it collided with the Erlangping arc attached to the sourthern margin of the NCB. Combining with previous results, we suggested that the NQ UHP terrane underwent fast exhumation from>120km to ca.30km within10Myr intervals. As a few examples of UHP metamporphism have been discovered in some accretionary orogens, accretionary orogenic belts may play important roles in recycling of continental crust into the manle besides generation of continental crust.(4) In situ zircon SIMS/LA-ICPMS methods revealed that different rock types of the Fushui complex had identical formation ages of488-484Ma, confirming the late Cambrian-early Ordovician magmatic activities. The Fushui complex belongs to shoshonitic series, and is characterized by extreme LILE enrichment (e.g. Ba, Pb and Sr), depletion of HFSEs (e.g. Nb, Ta, Zr, Hf, P and Ti), high initial Sr isotopic ratios (0.7100-0.7151), negative εNd(t)(-3.97to-5.68) and εHf(t) values (-2.85to0.34), positive△εHf values (3.85-5.37) as well as high zircon δ18O values (6.86±0.13‰). These geochemical features indicate that the mantle source has been metasomatized by subducted oceanic sediments. A simple two end-members mixing model constrained the amount of subducted sediments in the Fushui mantle source to～6%. The Fushui complex originated from1-6%equilibrium melting of a phlogopite-bearing garnet lherzolite by a model of non-modal melting. Shoshonitic magmas have been discovered in modern nascent arcs. Combined with previous studies on the tectonic evolution of the Qinling orogen, we suggest that the generation of the Fushui complex was induced by the subduction of the Paleotethyan Ocean when it jumped from the northern to the southern boundary of the NQ microcontinent.(5) Most zircons in three granulite samples and a gneiss sample from the NQ unit in the Tongbai orogen have core-rim structures. The cores have oval or rounded shapes, oscillatory zones, high Th/U ratios and variable U-Pb ages, suggesting a detrital genesis. The youngest group of the detrital zircons yields a weighted mean206Pb/238U age of450±5Ma, which is interpreted as the maximum depositional age of their protoliths. A major age cluster is ca.450-490Ma, corresponding to prolonged arc magmatism in the northern Tongbai and Qinling orogens. Another group of ages ranges from660to950Ma, implying that the NQ unit of the Tongbai orogen belongs to the SCB. Metamorphic zircons in the granulites have weak or no zoning, low Th/U ratios and REE contents, negative Eu anomalies, and relatively flat HREE patterns, which indicate they formed contemporaneously with garnet and plagioclase, and thus under granulite-facies metamorphic conditions. Their weighted mean age of424±4Ma is taken as the best estimated age of the granulite-facies metamorphism. Zircons in a leucosome sample have significant negative Eu anomalies and relatively flat HREE patterns with a formation age of428±4Ma, indicating partial melting coeval with the granulite-facies metamorphism. Metamorphic zircons in the gneiss show apparently negative Eu anomalies, and variable Th/U ratios and HREE contents. They yield a weighted mean206Pb/238U age of438±4Ma, which is suggested to record the timing of the prograde metamorphism. Zircon grains in a gabbro and a granodiorite yield U-Pb ages of432±4and424±4Ma, respectively, which are interpreted as their formation ages and identical to the ages of the granulite-facies metamorphism. They have positive εHf(t) values up to5.1, showing clear evidence for crustal growth during their formation. The appearance of the Silurian granulite-facies metamorphism and magmatism in the Tongbai orogen provides a good paradigm of coeval granulite-facies metamorphism, magmatism and crustal growth in an arc-continent collision orogenic belt.(6) Detailed geochronological and geochemical studies were performed on the Erlangping gabbroic rocks. U-Pb dating of zircons from two gabbroic diorite samples yielded two weighted mean ages of443±3and440±4Ma. The Erlangping gabbroic rocks are tholeiitic in composition with variable MgO (4.48-8.17%), and low K2O (0.25-1.01%) and TiO2(0.60-1.79%) contents. They are characterized by enrichment of LILEs, depletion of HFSEs, and highly positive whole-rock εNd(t)(3.76-5.68) and zircon εHf(t)(12.0-12.1) values. The geochemical features indicate that the Erlangping gabbroic rocks were derived from a relatively depleted lithospheric mantle that had been exposed to metasomatism by slab-derived fluid shortly before their generation. Their occurrence, coupled with coeval granitoids with highly positive εNd(t) values, manifests significant crustal growth in the North Qinling-Tongbai orogen during the late Ordovician-Silurian. In addition, the Erlangping gabbroic rocks have low Sr/Y (7.32-26.2) and LaN/YbN (1.34-4.07) ratios, but high HREEs, Y and Sc contents. This implies that their mantle source had no residual garnet, and thus partial melting of the lithospheric mantle occurred under relatively low-pressure conditions. The generation of the Erlangping gabbroic rocks might result from incipient rifting of the Erlangping back-arc basin induced by the northward subduction of the Paleotethyan Ocean.(7) Zircon SIMS/LA-ICPMS datings indicated that the Zhangjiadazhuang, Manziying, Xizhuanghe, and Sikeshu plutons formed at481,460,458and436Ma, respectively. The zircon crystals in the Zhangjiadazhuang, Manziying and Xizhuanghe plutons have highly positive εHf(t) and low δ18O values, whereas the Sikeshu pluton has slightly positive zircon εHf(t) and high δ18O values. These results combined with major and trace element data revealed that the Zhangjiadazhuang and Xizhuanghe plutons originated from partial melting of juvenile oceanic materials that had been altered by hydrothermal fluids under high tempreture conditions. The Manziying monzogranitic pluton was likely derived from partial melting of juvenile tonalities, while the Sikeshu pluton was the result of partial melting of old continental crust. In light of the regional geological evolution, we suggested the accreted oceanic terrane was one of the main sites for continental crustal growth.(8) Detailed zircon SIMS/LA-ICPMS U-Pb isotopic analyses revealed that the Taoyuan and Huanggang plutons in the Tongbai orogen formed simultaneously at ca.443Ma, spatially and temporally associated with the Erlangping gabbroic rocks. The two plutons have distinct geochemical features in term of whole-rock major and trace elments and Sr-Nd isotopes as well as zircon Hf-O isotopes. The Taoyuan pluton consists mainly of biotite granite and trondhjemite. They are rich in LILEs and depleted in HFSEs, and have highly positive whole-rock εNd(t) and zircon εHf(t) values as well as low zircon818O values, indicating they resulted from partial melting of juvenile oceanic crust that had been altered by hydrothermal fluids under high tempreture conditions. The Huanggang pluton is composed mainly of granodiorite, which is characterized by enrichment of LILEs, depletion of HFSEs, negative whole-rock εNd(t) and zircon εHf(t) values and high zircon δ18O values. The zircon εHf(t) values correlated negatively with the zircon δ18O values. One enclave sample in the Huanggang pluton has a high positive whole-rock εNd(t) value, which is distinct from that of the host granodiorite. These geochemical features indicate that the Huanggang pluton originated from partial melting of old continental crust coupled with direct addition of mantle-derived materials. Combining with regional geological history, we suggested the Erlangping intraoceanic arc had accreted into the southern margin of the NCB before ca.443Ma. During the opening of the Erlangping backarc basin at ca.443Ma, two types of growth of continental crust occurred simultaneously. |
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